The Belgian forward, who has scored 15 times this season, had played every minute in the Premier League before being taken off on a stretcher against Southampton on Saturday following a clash of heads with Wesley Hoedt.

That ruled Lukaku out of Monday's trip to his former club Everton and he is set to be absent for the FA Cup third-round tie with Derby on Friday too.

Romelu Lukaku was taken off on a stretcher during Saturday's 0-0 draw with Southampton

Speaking prior to the clash with the Toffees, Mourinho told Sky Sports: 'I would say a week (out for Lukaku).

'It's nothing really important - just a little protection, but no problem.'

There are 10 days between Derby's visit to Old Trafford and United's Premier League home clash with Stoke, when Lukaku may be available again.